Cross phase modulation (XPM) for modulating a refractive index and cross gain modulation (XGM) for modulating gain (or an absorption) have realized an optical device to control an optical signal with another light. A semiconductor optical amplifier is well known as an optical element used for the XPM or XGM.
For instance, by inputting a signal light to carry data and a CW light into an XPM or XGM optical element together, the CW light can be modulated by the signal light. This operation is used for changing optical carriers to carry data, namely used for wavelength conversion. Furthermore, by using one of the lights as a control light, an optical switch or an optical gate can be realized.
Wavelength converters using an SOA are described in the following patents:                U.S. Pat. No. 6,437,905;        U.S. Pat. No. 6,256,137;        U.S. Pat. No. 6,349,106;        U.S. Pat. No. 6,294,821;        U.S. Pat. No. 6,259,552;        U.S. Pat. No. 5,946,129;        U.S. Pat. No. 6,046,841; and        U.S. Pat. No. 5,959,764.In the U.S. Pat. No. 5,959,764, a wavelength converter is described that utilizes mutual absorption saturation characteristics of an electroabsorption optical modulator.        
FIG. 3 illustrates a schematic block diagram of a conventional wavelength converter using an SOA. FIG. 4 illustrates waveform examples of respective parts of the converter.
A pulse signal light 50 with a signal wavelength λs for carrying data enters a combiner 52. A waveform 70 in FIG. 4 shows a waveform example of the input pulse signal light 50. In the following explanation, a period in which an optical pulse exists in the pulse signal light 50 and its corresponding period are called as a pulse period and a period in which no optical pulse exists and its corresponding period are called as a non-pulse period.
A continuous wave (CW) light (probe light) 54 with a probe wavelength λp different from the signal wavelength λs enters a splitter 56. The splitter 56 splits the CW probe light 54 into two portions to apply one portion to the combiner 52 and the other to a phase adjuster 58.
The combiner 52 combines the input pulse signal light 50 and the one portion of the CW probe light 54 from the splitter 56 and applies the combined light to a semiconductor optical amplifier (SOA) 60. The SOA 60 modulates optical phase of the CW probe light 54 using the XPM according to amplitude variation of the pulse signal light 50. Here, assuming that the bias of the SOA 60 is set so that the optical phase of the CW probe light 54 differs by π between the pulse period and the non-pulse period of the pulse signal light 50.
An optical bandpass filter 62 passes through only the component of probe wavelength λp out of the output light from the SOA 60. A waveform 72 in FIG. 4 illustrates amplitude variation and phase variation of the output light (with the probe wavelength λp) from the optical bandpass filter 62. The SOA 60 also has an XGM effect and accordingly varies the amplitude of the CW probe light 54 according to the amplitude variation of the input pulse signal light 50. This variation becomes AM noise in the CW probe light 54. In particular, large AM noise is superposed in the center of the pulse period. This is because the gain of the SOA 60 for the wavelength λp decreases during the pulse period of the pulse signal light 50. Although omitted in FIG. 4, a pattern effect dependent on a data pattern of the pulse signal light 50 having entered the SOA 60 also causes the superposition of the AM noise on the CW probe light 54. Under an ideal condition that no XGM nor pattern effect exists, the amplitude of the output light from the optical bandpass filter 62 flattens and only the optical phase of the output light varies according to the amplitude variation of the pulse signal light 50.
The phase adjuster 58 adjusts the optical phase of the CW probe light 54 from the splitter 56 so that the optical phase shifts relatively by 1 from optical phase of a mark part of the output from optical bandpass filter 62.
A combiner 64 combines the output light from the optical bandpass filter 62 with the CW probe light which optical phase was adjusted by the phase adjuster 58. A waveform 74 in FIG. 4 illustrates a waveform of an output light 66 from the combiner 64. The output light 66 from the combiner 64 is a wavelength-converted signal light in which the wavelength λs of the input pulse signal light 50 was converted into the probe wavelength λp.
In the conventional configuration, as illustrated in the waveform 74 in FIG. 4, the AM noise generated through the XGM in the SOA 60 is superposed on the wavelength-converted signal light (the output light 66 from the combiner 64). Furthermore, AM noise of a longer cycle generated through the pattern effect from the data pattern of the input pulse signal light 50 is also superposed on the output light 66 from the combiner 64. Such AM noise greatly deteriorates the transmission characteristics and accordingly makes the data transmission unstable.
This type of problem generally occurs not only in a wavelength converter but also in an optical device in which interaction from the XPM of two signal lights having a different wavelength is utilized and in which AM noise caused by the XGM and/or pattern effect is superposed.